Two axis gas bearing accelerometer

ABSTRACT

A new type of rugged two axis gas bearing accelerometer wherein the gas bearing is piezo-electrically actuated, the mass sensor is a simple flat metal washer which is suspended between two sets of piezo-electric ceramic parts, the pick off is a conventional capacitance pick off and the forcer principle is electrodynamic.

United States Patent Marggraf et al. i451 June 27, 1972 [541 TWO AXISGAS BEARING [561 References cmd ACCELEROMETER UNrrED STATES PATENTS3,520,197 7/1970 Blanding et al ..73/516 [72] Inventors: Kurt A.Mlrggrd, Tonawanda; Ernest 3 237 456 1966 h J 7 Metzler Euensviue bothofN'Y. 3/ S aw, r. 3/503 [73] Assignee: The United States of America nsPrimary ExamnU-Richafd C. @Heisser "pl-muted by the sea-wy of the Ah-Assstant Examiner-Herben Goldstein )vom l Attorney-Harry A. Herbert, Jr.and Ruth G. Codier [22] Filed: June 2, 1970 [57] ABSTRACT [21] APPLNQ;42,647 A new type of rugged two axis gas bearing accelerometer whereinthe gas bearing is piezo-electrically actuated, the mass sensor is asimple flat metal washer which is suspended [52] US' Cl? "73,516 Rbetween two sets of piezo-electric ceramic parts, .the pick off [5l lInt' Cl' ---G01P 15/08 is a conventional capacitance pick o' and theforcer principle [58] Field el Search ..73/516, 517 is electrdynamia 1cnam, z Drawing mms PT'ENTEDJUHN |972 SHEET 2 0F 2 j BY TIRE TWO AXISGAS BEARING ACCELEROMETER BACKGROUND OF THE INVENTION This inventionrelates to a two axis gas bearing accelerometer and particularly to agas bearing which is piezo-electrically actuated, and wherein the sensormass is a simple flat metal washer which is suspended between two setsof piezo-electric ceramic parts, the pick off is a conventionalcapacitance pick off and the forcer principle is electro-dynamic.

The equipment available is a single axis accelerometer which iscomplicated, extremely delicate to manufacture and maintain and thereforextremely costly. The factors which contribute to the high cost can besummarized as follows: (1) complexity-many parts and many types ofparts; (2) unique parts which are complex in themselves and thereforedifficult and expensive to fabricate; (3) exotic materials which arecostly and require special handling; (4) precision parts requiringextreme accuracy in fabrication, resulting in low yield and high cost;(5) manufacturing and assembly processes which require state-of-the-arttype control.

The accelerometer now available comprises, in general, six systems, eachsystem being a delicate and complex structure.

The six systems which constitute the single axis accelerometer of theprior art, and each of which the invention proposes to simplify are asfollows:

l Proofmass-or sensor mass disc The presently available sensor mass is acomplex beryllium cylindrical proofmass which has to have legs. A finewire coil is wound on cylinder with terminals soldered to suspensionsprings. Both inside diameter, inside flange, and groove for coil haveto be accurately held. The soldering process is delicate.

2. Suspension System Two delicate springs are required to be soldered,by a critical process, to the sensor mass legs. Manufacturing of thesesprings is a precision operation. One end of springs have to be attachedto the mounting base, with electrical insulation provision, since thesuspension springs also serve as electrical connection for the coil.Cross-coupling of suspension force is extremely critical as to thecentered or null position ofthe sensor mass. Maintaining zero forcecross-coupling from the suspension spring into sensitive axis subsequentto temperature cycles presents yield (cost) problem.

3. Pick Off System Capacitive pick off consists of two capacitive pick orings precisely positioned with respect to proofmass, depending on nullforce, position of suspension system. Fairly involved attachment andadjustment procedure.

, 4. Forcing System Permanent Magnet DC Forcing The magnets areexpensive and require special aging and handling procedures. Their heavyweight requires special mounting provisions to withstand vibration andshock. The magnets also require accurate placement. Magnet strength isalso temperature sensitive and reduces as a function of time.

5. Case All parts of the instrument are mounted directly to the castprecision machined case. lt is complex with many precision surfaces,holes, and diameters.

6. Temperature Control Even with temperature compensation, a temperaturecontrol system is required because of temperature sensitivity of themagnets.

SUMMARY OF THE INVENTION lt is the object of the present invention toprovide a two axis accelerometer comprised of five of the six systemsabove noted. Each of the five systems are greatly simplied, most of themhaving no precision requirements and presenting no spe` cial problems ineither manufacture or assembly. The sixth system which involvestemperature control is dispensed with altogether.

l. The proofmass or sensor disc, in place of the complex systemheretofor available, is a one piece disc, preferably of aluminum with asegmented flange. The only important accuracy necessary is the flatnessof the disc.

2. The suspension system for the proofrnass is a PiezoElectric-vibratory suspension which requires no physical or electricalconnection to the proofmass. The piezo-electric suspension rings aresimple to manufacture. Only one surface of each ring required accurategrinding. The air gap, set by special technique, requires nomanufacturing process because of the characteristics of thepiezo-electric crystal.

3. The pick ofi` system comprises a cylindrical tube with metalized andsegmented inner-diameter surface. This pick off cylinder has no tighttolerance requirements.

4. Eddy current forcing system, using AC. excitation, is inherentlydigital. Excitation coil for the primary current is mounted on a centerpart and has no special tolerance requirements. The two forcerassemblies are also simple and require no special tolerances. The gapbetween opposing forcer coils is important and is held constant,independently of temperature, by the proper design of the case. The needfor a temperature control system being thereby eliminated.

5. The case which houses the above noted elements is compsed of a pairof E-Cores which serve the dual function of mounting all elements of theaccelerometer including part of the magnetic circuit. The E-Coreelements are standard ferrite moldings, and only the facing surfaces ofeach have to be ground.

6. As before noted, no temperature control is required. The instrumenthas potentially instantaneous reaction from a cold Start.

These and other advantages, features and objects of the invention willbecome `more apparent from the following description taken in connectionwith the illustrative embodiments in the accompanying drawings. v

DESCRIPTION OF THE DRAWING FIG. l is a cross sectional view of the twoaxis accelerometer, with portions omitted for clarity.

FIG. 2 is an exploded view of the component parts of the device.

DESCRIPTION OF THE PREFERRED EMBODIMENT The accelerometer is indicatedgenerally by the numeral l0. The sensor mass l2 is a flat metal washerwith a circular opening 14 located at its center. The segments 16 of acylindrical flange are integral with the sensor mass disc l2 and arelocated around its outside circumference. The accurate perpendicularityof the segments 16 is not crucial from a precision point of view.

In operation the sensor mass l2 is suspended in a vibrating gas bearingand is free to move in any direction within the plane of the disc l2.The sensor mass l2 is thus sensitive to accelerations along two inputaxes and is therefore a dual axis device.

This is accomplished by suspending the sensor disc 12 in a vibrating gasbearing between two segmented piezo-electric crystal rings 18 and 20which provide vibratory excitation. The rings 18 and 20 are mounted in,but electrically isolated from, the ferrite outer case 28-30 in whichthe elements of the accelerometer are mounted. The rings 18 and 20 arepreferably ceramic, with metalized faces. The gas gaps between theceramic crystal faces of the elements 18 and 20 and the sensor disc 12can be made very rigid to suspend the sensor mass l2 against anypractical vibration or steady state acceleration or shock, or thesuspension capability may be adjusted to a strength merely capable ofsuspending the sensor mass l2 in a near zero gravity field. A simplechange ofthe excitation voltage effects a modification in the suspensioncapability. The operation is instantaneous and has low powerrequirements. It will be apparent that delicate spring systems and theassociated problems of fatigue and wearing are dispersed with. Thesensor disc 12 is free floating, requiring no attachments. The expansioncharacteristic of the crystals 18 and 20 under applied voltage offer asimple means of adjusting the space gap.

The pick off system where function is to sense the position of thesensor mass comprises the flange segments 16 previously described asperpendicular segments attached to the circumference of the sensor disc12, in conjunction with a cylindrical sleeve 24. The sleeve 24 isrigidly mounted in the outer case 28-30. Any displacement of the sensordisc 12 generates two signal components.

The inside of the pick off cylinder 24 has four equal quadrants,indicated at 26 each plated with a metallic surface. No specialprecision requirement is imposed on the cylinder 24 since the suspensionforces are largely independent of the position ofthe sensor mass.

Four capacitors 26 are thus produced with the proofmass or sensor discl2 as the common plate. The diametrically opposite capacitors 26 areconnected in a wheatstone capacitance bridge. As the sensor disc 12moves along an input axis, one capacitance is increased while theopposite capacitance is decreased. This produces an unbalance in thebridge that is proportional to the displacement of the sensor mass 12from centered or null position.

The Forcing System is based on the forcer principle which iselectro-dynamic; that is, a forcer magnetic field density inter fereswith a steady current flow in the sensor disc 12.

The sensor disc or proofmass l2 is rebalanced by an eddy current forcingsystem which requires no physical connections to the proofmass 12,eliminating costly and delicate equipment or bulky permanent magnets. Aspecial temperature compensation scheme has been devised which makes theforcing independent of the temperature.

In order to generate the current in the sensor l2 without use ofconductor leads, a transformer configuration is used which induces thenecessary sensor current without applying force to it.

The magnetic transformer structure is composed of two E cup cores 28 and30 which serves also the function of a ferrite shell for mounting all ofthe elements of the device. The shell elements 28 and 30 are formed withmating central core elements 32 and 34 which form a central post aroundwhich are wound an excitation coil. See FIG. 1.

The sinusoidal primary magnetic field cuts through the sensor disc 12periodically and generates the sensor current. The current value isindependent of the sensor position because of an enclosed secondary turn(not shown) which is cut by the total primary field.

The forcer coil assembly is comprised of two complementary capacitiveelements 40 and 42. Two sets of four poles each (see 44 FIGS. 1 and 2)are thus aligned with the respective sets of capacitors in the spacearound the center pole excitation coils 36 and 38, forcer coils 40 and42 surround each pole 44.

An alternating current input applied to one set of forcer coils 40generates a forcer field density between the pole faces which in turninterferes with the secondary current flowing in circular paths in thesensor element l2.

The whole package is mounted on a plate 48. The plate 48 and a cover 50are provided with interengaging shoulder and rim elements. A securepackage is obtained. This type of forcer is adaptable to three differentkinds of feedback modes:

Mode l is an amplitude modulated (analogue) operation,

where the phase of the forcer field is kept equal to the sensor currentphase, but the amplitude is always proportional to the pick-ofi` signal.

Mode 2 is phasemodulated (also analogue) operation, where the amplitudeof the forcer field is kept constant but the phase of this field isbiased proportional to the pickoff signal.

Mode 3 is a time modulated operation (digital) where the amplitude ofthe forcer field is always constant and the forcer phase always equal orin opposition to the sensor current phase, which depends on the polaritof the acceleration input. But this condition exists only as ong as theacceleration sensor mass is deflected to or beyond the trigger level bythe input acceleration.

The trigger level initiates a burst of full sine waves to counteract theacceleration input, which caused the sensor to be shifted to the triggerlevel.

The number of counted sine waves per unit of time is then proportionalto the input acceleration, while each counted sine wave represents onevelocity increment.

In a fluid lled accelerometer, a smooth operation is guaranteed becauseof sufficient damping. Sufficient damping is not natural in a gas or airfilled accelerometer. An artificial damping is made by differentiatingthe demodulator output signal, process the DC damping signal to anamplitude modulated forcer signal, which is superimposed to theconstrainment signal of any mode it may be.

The two halves of an accelerometer are made semi-solid by a filling ofall empty spaces with a slightly flexible Epoxy or a silicon rubber,which can encapsulate also the vibrating piezoelectric ceramics withoutdetermining their function.

The operation of this accelerometer is adaptable to a variety ofrequirements in terms of constrainment mode, acceleration ranges, andsliding range operation.

Although the invention has been described with reference to a particularembodiment, it will be understood to those skilled in the art that theinvention is capable of a variety of alternative embodiments within thespirit and scope of the appended claims.

I claim:

l. In a dual axis accelerometer having a flat disc sensor element and avibratory suspension system for free suspension of said sensor element,the improvement comprising a plurality of piezo-electrically operatedcrystals located on either side of said sensor element for providingvibrating exitation thereto, a capacitive pick off system comprising asegmented cylindrical flange on the outer periphery of said sensorelement and a cylinder located circumferentially spaced from said sensorelement, and an eddy current forcing system comprising a pair ofexcitation coils wound around a central post which passes through acentral opening in said sensor element, and two forcer assemblies, oneof said forcer assemblies being located on either side of said sensorelement and each comprising four pole pieces, a forcer coil on eachindividual one of said forcer pole pieces, a pair of identical ferriteE-Cores operating as a casing for mounting and containing all of theelements of said forcing System and in addition forming the flux returnpath to complete the forcing magnetic circuit, the inwardmost portionsof said E-cores providing the central post around which said excitationcoils are positioned, thereby providing an excitation source for themagnetic circuit and the rebalance force path to said sensor elementwithout any physical connection therewith.

1. In a dual axis accelerometer having a flat disc sensor element and avibratory suspension system for free suspension of said sensor element,the improvement comprising a plurality of piezo-electrically operatedcrystals located on either side of said sensor element for providingvibrating exitation thereto, a capacitive pick off system comprising asegmented cylindrical flange on the outer periphery of said sensorelement and a cylinder located circumferentially spaced from said sensorelement, and an eddy current forcing system comprising a pair ofexcitation coils wound around a central post which passes through acentral opening in said sensor element, and two forcer assemblies, oneof said forcer assemblies being located on either side of said sensorelement and each comprising four pole pieces, a forcer coil on eachindividual one of said forcer pole pieces, a pair of identical ferriteE-Cores operating as a casing for mounting and containing all of theelements of said forcing system and in addition forming the flux returnpath to complete the forcing magnetic circuit, the inwardmost portionsof said Ecores providing the central post around which said excitationcoils are positioned, thereby providing an excitation source for themagnetic circuit and the rebalance force path to said sensor elementwithout any physical connection therewith.